Odd behavior that took me a bit to figure out exactly what was going on. If using RDP on a client MAC, the modifier keys do nothing (i.e. cannot type capital letters, alt-tab, etc.). Switching back to bluetooth for preferred channel, the modifiers start working again. This appears to only happen with RDP as I checked a number of other apps (even a couple that similarly connect to remote windows systems) and did not have the same issue. Don't know that there's much to be done about this (other than use it with bluetooth rather than network), but this might be useful to other users having the same problem.
On macOS, there are several event Taps where an application can inject keyboard events or monitor keyboard events. They are located from lower-level (near OS kernel) to upper-level (near applications). Keyboard events travel from lower-level to upper-level. across Client injects keyboard events into a mid-level event Tap. If an application like your RDP is monitoring on a lower-level event Tap, it will not see the keyboard event that across injects. We will try to change the injection event Tap to a lower one, but it won't guarantee that your RDP must see the injected event since an application can monitor keyboard events from the lowest level where across Client is unable to inject keyboard events. Aside from the network channel, Bluetooth input is just Bluetooth keyboard input. Your across Server Mac is working as a Bluetooth keyboard/mouse combo. Therefore, it must work the same as a general Bluetooth keyboard. That is the difference between network and Bluetooth input channels. I hope it helps your understanding.
Had mostly put this here in case another user came across the same. In the case of RDP only the shift, alt, ctrl, function, and windows key (mapped from mac command key in my case) wouldn't work in network mode. All other keys did. Your response jogged my memory that RDP has a keyboard input setting. Changing that to unicode rather than scancode works like a charm.
Actually, looks like I spoke too soon. The shift key works when set to unicode, but others do not. Oh well, maybe that at least provides some additional input into what this app is actually doing.
